High Purity Materials for Photovoltaics
Poly- or Multicrystalline Silicon
Advantages of Crystalline Silicon
Copper Indium Gallium Selenide CIGS
Advantages of CIGS
Cadmium Telluride - CdTe
Advantages of CdTe
A photovoltaic substance is a material used in the creation of solar cells that convert sunlight directly into electricity. The long-term goal of photovoltaic (PV) devices has been to reduce our dependency on fossil fuel generated electricity.
The main benefits of electricity produced by PV’s are:
- Reliable operating systems
- Low operating costs
- Virtually zero environmental impact
- Modular design
Presently, most PV solar cells are produced from either single- or polycrystalline silicon with efficiencies of around 13-17%. This first generation of photovoltaics is heavily reliant on the supply of pure silicon in the form of single crystals. Technology has now been developed to produce polycrystalline silicon, specifically for the PV industry, with lower purity and lower costs. However, the main disadvantage of this material is the lower cell efficiency.
Therefore, critical to the long-term commercial success of PV technologies are advances in module efficiencies and improvements in cost and reliability. In an effort to circumnavigate these issues, two emerging technologies have developed based on either Copper Indium Gallium Selenide (CIGS) or Cadmium Telluride (CdTe) solar cell devices. The advantages of these thin-film PV’s include lower cost, reduced material usage, lower energy requirement for manufacture and shorter payback time.
By far, the most prevalent bulk material for solar cells is crystalline silicon, also known as solar grade silicon. Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon or wafer.
Often made using the Czochralski process, single crystal wafer cells can be expensive. Since they are cut from cylindrical ingots, they do not typically cover a square solar cell module without a substantial waste of refined silicon. Therefore, most monocrystalline silicon panels have uncovered gaps at the four corners of the cell.
This type of monocrystalline silicon is formed by drawing flat thin films from molten silicon and has a multicrystalline structure. Although less efficient, ribbon silicon cells are more cost effective because their production does not require sawing from ingots.
Large blocks of molten silicon are carefully cooled and solidified to create cast square ingots. Less expensive to produce than monocrystalline silicon cells, multicrystalline silicon cells are also less efficient.
The advantages of using crystalline silicon include:
- Up to 30% efficiency
- Long life
Unlike silicon solar cells which are based on a p-n junction, the structure of CIGS is a complex heterojunction.
Photovoltaic devices based on Cu(In,Ga)Se2 (CIGS) perform with 19% efficiency, the highest of any thin film technology. However, the possibilities for even higher performance are significant.
Typically, CIGS cells use either a glass, steel or polymer substrate, onto which a layer of molybdenum is sputtered (Back Contact). The 1.3-2.5 mm thick CIGS layer can be deposited in various ways including sputtering, co-evaporation, printing and electroplating.
A thin cadmium sulfide buffer is added next, prepared by chemical bath deposition. It is then over-coated with a transparent conductor (aluminium doped zinc oxide).
Some of the advantages of using CIGS include:
- Modules can be on glass, flexible steel or polymer substrates
- Able to produce lower-cost PV with less material usage
- Highest efficiency of thin-film technologies: 19.9% for single cells, 13% for large modules
Cadmium Telluride (CdTe) is an ideal semiconductor for use in photovoltaic applications, as the band gap (about 1.5 eV) almost perfectly matches the solar spectrum.
Thin-films of CdTe can be deposited onto a substrate by a variety of different methods, including evaporation, electrodeposition, and printing. Photovoltaic modules based on thin-film cadmium telluride technology are of great interest for large-scale grid connected applications, primarily due to cost benefits over other PV technologies.
Advantages of using CdTe are:
- Simplified manufacturing
- Abundant supply of cadmium produced as a by-product of industrial metals such as zinc
- Absorbs sunlight at close to the ideal wavelength
- 10.6% efficiency for modules
Alfa Aesar produces a range of high purity materials specifically for the photovoltaic industry. The following is a representative listing only; if you require a specialized compound or metal, please visit Alfa Aesar.
To request literature on high purity silicon and cadmium telluride materials for photovoltaics, please visit the Alfa Aesar website.
Source: Alfa Aesar
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